diff --git a/Dockerfile b/Dockerfile
index 6da7adc..0c69314 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,6 +15,7 @@ ENV GEM_HOME=/app/bundle
 ENV PATH="${GEM_HOME}/bin:${PATH}"
 
 COPY --chown=asxpio Gemfile Gemfile.lock* ./
+ENV BUNDLE_FROZEN=true
 RUN bundle install --without development:test
 
 COPY --chown=asxpio . .
@@ -24,4 +25,8 @@ ENV PUMA_THREADS="4:16"
 ENV RACK_ENV=production
 EXPOSE 3000
 
+# Slim image has no curl; ruby is right there.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD ruby -rnet/http -e 'port = ENV.fetch("PUMA_PORT", "3000"); exit(Net::HTTP.get_response(URI("http://127.0.0.1:#{port}/healthz")).code == "200" ? 0 : 1)'
+
 CMD ["sh", "-c", "exec puma -b tcp://0.0.0.0:${PUMA_PORT} -t ${PUMA_THREADS} --preload"]